Investigating the origin of optical and X-ray pulsations of the transitional millisecond pulsar PSR J1023+0038

Abstract

Context. PSR J1023+0038 is the first millisecond pulsar that was ever observed as an optical and UV pulsar. So far, it is the only optical transitional millisecond pulsar. The rotation-and accretion-powered emission mechanisms hardly individually explain the observed characteristics of optical pulsations. A synergistic model, combining these standard emission processes, was proposed to explain the origin of the X-ray/UV/optical pulsations. Aims. We study the phase lag between the pulses in the optical and X-ray bands to gain insight into the physical mechanisms that cause it. Methods. We performed a detailed timing analysis of simultaneous or quasi-simultaneous observations in the X-ray band, acquired with the XMM-Newton and NICER satellites, and in the optical band, with the fast photometers SiFAP2 (mounted at the 3.6 m Telescopio Nazionale Galileo) and Aqueye+ (mounted at the 1.8 m Copernicus Telescope). We estimated the time lag of the optical pulsation with respect to that in the X-rays by modeling the folded pulse profiles with two harmonic components. Results. Optical pulses lag the X-ray pulses by ∼ 150 µs in observations acquired with instruments (NICER and Aqueye+) whose absolute timing uncertainty is much smaller than the measured lag. We also show that the phase lag between optical and X-ray pulsations lies in a limited range of values, δφ ∈ (0 − 0.15), which is maintained over timescales of about five years. This indicates that both pulsations originate from the same region, and it supports the hypothesis of a common emission mechanism. Our results are interpreted in the shock-driven mini pulsar nebula scenario. This scenario suggests that optical and X-ray pulses are produced by synchrotron emission from the shock that formed within a few light cylinder radii away (∼ 100 km) from the pulsar, where its striped wind encounters the accretion disk inflow.

Timing Analysis of the 2022 Outburst of the Accreting Millisecond X-Ray Pulsar SAX J1808.4-3658: Hints of an Orbital Shrinking

“…The black widow PSR J2051−0827 exhibits orbital variability characterized by sinusoidal modulation with changing amplitude (see Figure 5 from Shaifullah et al 2016). A chaotic orbital evolution has been also observed in the transitional redback system PSR J1023 + 0038 during its radio pulsar state (Archibald et al 2015), while the orbital period variations seem to be smoother in the X-ray active state (Jaodand et al 2016;Papitto et al 2019;Burtovoi et al 2020;Illiano et al 2023). The long-term orbital modulation of a few black widow pulsars (Applegate & Shaham 1994;Arzoumanian et al 1994;Doroshenko et al 2001) has been interpreted in terms of gravitational quadrupole coupling (GQC) model (Applegate 1992;Applegate & Shaham 1994).…”

The Third Fermi Large Area Telescope Catalog of Gamma-Ray Pulsars

“…We also folded the post-transition data using a timing model built from X-ray timing by Jaodand et al (2016) and X-ray and optical timing from Illiano et al (2023). We fit a polynomial function to the orbital phase deviations measured by Illiano et al (2023), and folded the LAT data using this orbital model and the spindown model of Jaodand et al (2016). No pulsations were detected.…”

Quantum Astronomy at the University and INAF Astronomical Observatory of Padova, Italy

“…This pulsar, too, is observable in the UV and X-ray bands. As described in [35], (quasi-)simultaneous observations were obtained with AquEye+ and the XMM-Newton and NICER X-ray instruments. These observations, and in particular those acquired with AquEye+ and NICER, pointed out that the optical pulses follow the X-ray pulses by ∼150 µs, with an uncertainty that is much smaller than the measured time shift.…”